The field to which the present invention relates is that generally known as electrical discharge machining, sometimes hereinafter referred to as EDM, in which material is removed from an electrically conductive workpiece by the action of electrical gap discharges occurring between a tool electrode and a workpiece. A dielectric coolant fluid is circulated and recirculated through the gap, usually under pressure, throughout the machining operation. An electrode or a workpiece servo feed system is used to provide relative movement and thus maintain an optimum gap spacing between the electrode and the workpiece as the workpiece material is being removed.
It is important to the process of EDM that the machining power pulses provided at the gap give closely and precisely controllable on-off time and frequency to insure repeatability of results and to provide appropriate cutting action for the type of operation being carried on. For example, with a roughing type operation, relatively low frequency, high current magnitude pulses would be used. With a finishing type machining operation, relatively high frequency, low current magnitude pulses would be used. Various types of pulse generators which have this capability have been developed and are in commercial use for EDM. One commonly used type of EDM power supply includes as a principal part of its machining power pulse generator an astable multivibrator in which on-off time and frequency are controlled and preset by a ganged capacitor and resistor arrangement. One example of this type of pulse generator is shown and described in Kurt H. Sennowitz, U.S. Pat. No. 3,649,802, issued on Mar. 14, 1972 for "Protective System for Electrical Discharge Machining Power Supply Circuit", which patent is of common ownership herewith. Other types of pulse generators are in use which include various arrangements and combinations of astable multivibrators, oscillators and the like to provide for machining power pulses of controllable duration and frequency.
More recent developments in EDM include digital type pulse generators such as the one shown and described in Oliver A. Bell, Jr., U.S. Pat. No. 3,809,848, issued on May 7, 1974 for "Digitally Controlled Power Supply for Electrical Discharge Machining Apparatus". This patent has reference to a pair of presettable counters using thumbwheel switches in which the on-time switch and off-time switch and associated counters are sequentially enabled. A data selector stage is required in the circuit to complete the operation.
A similar arrangement for a digital type EDM pulse generator is shown and described in Oliver A. Bell, Jr., U.S. Pat. No. 3,809,847, issued on May 7, 1974, for "Method and Apparatus for Electrical Discharge Machining".
A still further type of digital multivibrator is shown and described in Oliver A. Bell, Jr., U.S. Pat. No. 4,071,729, issued on Jan. 31, 1978, for "Adaptive Control System and Method for Electrical Discharge Machining". This patent shows an on and off time generator which receives inputs from a programmable computer and from this general arrangement provides machining power pulses to the machining gap. All the above noted patents are of common ownership herewith.
The present invention shows a digital type pulse generator that is particularly adaptable to control by a programmable computer or similar input device. The present invention further discloses a pulse generator in which the setting can be changed under computer control to operate and receive a new on or off time input without turning off the power supply and restarting it.